The present invention relates generally to digital signal detector arrangements and in particular to baseband filter arrangements in wireless systems.
Wireless technology provides many applications for voice and/or data transmission. Today's cell phone networks offer a plurality of services for their customers including digital data services, such as digital email, Internet access, etc. In future applications, such as third generation wireless networks, a plurality of new digital data services will be provided. In particular, Internet applications will be highly improved and made more practical, for example, via high speed digital data transmission. Other digital data application, not yet applicable in today's wireless transmission technology, will be adapted and implemented.
High speed wireless data applications require high data throughput at a significantly lower bit error rate than voice applications. Bit errors in voice applications are usually easy to recover or do not need to be fully recovered due to redundancy capabilities of the human ear; whereas, digital data application often highly rely on the correctness of the submitted data. The quality of data transmissions in a digital environment highly depends on the quality of the transmission channel. Under severe channel conditions, the mobile device throughput is markedly affected due to retransmission of erroneous data packets, thus affecting the entire network throughput. This situation may be ameliorated by the use of antenna diversity and more sophisticated signal processing algorithms.
According to the prior art, decision feedback equalizers are used to compensate for the effects of the transmission channel, which can vary depending on the environment. A basic decision feedback equalizer (DFE) has a forward filter, a feedback filter, and a decision device. Decision feedback equalizers are effective against severe intersymbol-interference. Intersymbol-interference is an effect which creates distortion of the transmitted signal in a specific way. In a sequence of positive and negative symbol pulses, intersymbol-interference is the distortion of a symbol pulse within a particular symbol period caused by the smearing or spillover of symbol pulses of preceding and/or succeeding adjacent symbol pulses into the particular symbol period. The spillover of the preceding and/or succeeding symbol pulses will add to or subtract from the symbol pulse in the particular symbol interval depending upon whether the adjacent interfering symbol pulses are positive or negative in value. In applications with mobile devices, intersymbol-interference occurs due to the multi-path profile of the mobile channel, as well as the above mentioned smearing which is generated due to analog filtering. Unlike linear equalizers, decision feedback equalizers' decision errors propagate in the feedback branch thus affecting the outcome of future bit decisions.
A baseband filter is usually used to limit the signal fed into the equalizer to a predefined frequency band. Different types of interference exist in a wireless network. Co-channel interference is interference caused by two or more transmissions occurring simultaneously in the same channel. In a digital wireless telephone networks, such as GSM networks, the co-channel interference is mainly caused by the spectrum allocated for the system being reused multiple times (“frequency reuse”). The problem may be more or less severe, depending on the reuse factor, but in all cases, a signal received by a handset will contain not only the desired forward channel from the current cell but also signals originating in more distant cells. Adjacent channel interference is caused by extraneous power from a signal in an adjacent channel. Thus, the adjacent channel spills over part of its signal transmission into the used channel and thereby distorts the transmission within that channel. To prevent this kind of interference, different filter characteristics are used to prevent a decreased transmission quality. The choice of the baseband filter is generally a trade off between co-channel and adjacent channel interference rejection. Thus, systems either use a filter having the characteristics which favor co-channel or adjacent channel interference or use a compromise filter.
From the above, it is desirable to provide a filter arrangement for a digital signal detector arrangement which is more flexible.